As the world's population expands and its economy increases, the increase in the atmospheric concentrations of carbon dioxide is warming the earth causing climate changes. However, the global energy system is moving steadily away from the carbon-rich fuels whose combustion produces the harmful gas. Experts say atmospheric levels of carbon dioxide may be double that of the pre-industrial era by the end of the next century, but they also say the levels would be much higher except for a trend toward lower-carbon fuels that has been going on for more than 100 years. Furthermore, fossil fuels cause pollution and are a causative factor in the strategic military struggles between nations. Furthermore, fluctuating energy costs are a source of economic instability worldwide
In the United States, it is estimated, that the trend toward lower-carbon fuels combined with greater energy efficiency has, since 1950, reduced by about half the amount of carbon spewed out for each unit of economic production. Thus, the decarbonization of the energy system is the single most important fact to emerge from the last 20 years of analysis of the system. It had been predicted that this evolution will produce a carbon-free energy system by the end of the 21st century. The present invention is another product which is essential to shortening that period to a matter of years. In the near term, hydrogen will be used in fuel cells for cars, trucks and industrial plants, just as it already provides power for orbiting spacecraft. But, with the problems of storage and infrastructure solved (see U.S. application Ser. No. 09/444,810, entitled “A Hydrogen-based Ecosystem” filed on Nov. 22, 1999 for Ovshinsky, et al., which is herein incorporated by reference and U.S. patent application Ser. No. 09/435,497, entitled “High Storage Capacity Alloys Enabling a Hydrogen-based Ecosystem”, filed on Nov. 6, 1999 for Ovshinsky et al., which is herein incorporated by reference), hydrogen will also provide a general carbon-free fuel to cover all fuel needs.
Hydrogen is the “ultimate fuel.” In fact, it is considered to be “THE” fuel for the future. Hydrogen is the most plentiful element in the universe (over 95%). Hydrogen can provide an inexhaustible, clean source of energy for our planet which can be produced by various processes. Utilizing the inventions of subject assignee, the hydrogen can be stored and transported in solid state form in trucks, trains, boats, barges, etc. (see the '810 and '497 applications).
A fuel cell is an energy-conversion device that directly converts the energy of a supplied gas into an electric energy. Researchers have been actively studying fuel cells to utilize the fuel cell's potential high energy-generation efficiency. The base unit of the fuel cell is a cell having an oxygen electrode, a hydrogen electrode, and an appropriate electrolyte. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines and power supply applications of all sorts. Despite their seeming simplicity, many problems have prevented the widespread usage of fuel cells.
Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically air or oxygen, are supplied and the reaction products are removed, the cell continues to operate.
Fuel cells offer a number of important advantages over internal combustion engine or generator systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
The major components of a typical fuel cell are the hydrogen electrode for hydrogen oxidation and the oxygen electrode for oxygen reduction, both being positioned in a cell containing an electrolyte (such as an alkaline electrolytic solution). Typically, the reactants, such as hydrogen and oxygen, are respectively fed through a porous hydrogen electrode and oxygen electrode and brought into surface contact with the electrolyte. The particular materials utilized for the hydrogen electrode and oxygen electrode are important since they must act as efficient catalysts for the reactions taking place.
In a hydrogen-oxygen alkaline fuel cell, the reaction at the hydrogen electrode occurs between hydrogen fuel and hydroxyl ions (OH−) present in the electrolyte, which react to form water and release electrons:H2+2OH−->2H2O+2e−.At the oxygen electrode, oxygen, water, and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen and form hydroxyl ions (OH−):O2+2H2O+4e−->4OH−.The flow of electrons is utilized to provide electrical energy for a load externally connected to the hydrogen and oxygen electrodes.
The catalyst in the hydrogen electrode of the alkaline fuel cell has to not only split molecular hydrogen to atomic hydrogen, but also oxidize the atomic hydrogen to release electrons. The overall reaction can be seen as (where M is the catalyst):M+H2->2MH->M+2H++2e−.Thus the hydrogen electrode catalyst must efficiently dissociate molecular hydrogen into atomic hydrogen. Using conventional hydrogen electrode material, the dissociated hydrogen atoms are transitional and the hydrogen atoms can easily recombine to form molecular hydrogen if they are not used very quickly in the oxidation reaction.
In a zinc-air fuel cell, a type of metal-air fuel cell, the reaction at the anode occurs between the zinc contained in the anode and hydroxyl ions (OH−) present in the electrolyte, which react to release electrons:Zn->Zn+2+2e−At the cathode, oxygen, water, and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen and form hydroxyl ions (OH−):O2+2H2O+4e−->4OH−.The flow of electrons is utilized to provide electrical energy for a load externally connected to the hydrogen and oxygen electrodes.
Fuel cells, when used to power vehicles, are often used with an auxiliary battery pack because of the general inability of fuel cells to provide power instantly upon start-up or provide increased bursts of power for sudden acceleration. Such vehicles are generally termed hybrid electric vehicles (HEV). The auxiliary battery supplements the fuel cell power output during conditions requiring high power output, such as during start-up or sudden acceleration. PEM fuel cells do not work very well at low temperatures owing to the increase in the membrane resistance within the fuel cell at lower temperatures. In addition, the normal start up time required for the PEM cell even at ambient temperatures is quite significant making instant start difficult. Unlike PEM fuel cells, alkaline fuel cells are able to operate at ambient temperatures, since they do not use any membranes and the electrolyte does not freeze at temperatures above −60° C. Alkaline fuel cells, however, still require the use of a battery during instant start-up and sudden acceleration.
Hybrid systems have been divided into two broad categories, namely series and parallel systems. In a typical series system, a battery powers an electric propulsion motor which is used to drive a vehicle and an internal combustion engine is used to recharge the battery. In a parallel system, both the internal combustion engine and the battery power in conjunction with an electric motor can be used, either separately or together, to power a vehicle. In these types of vehicles, the battery is usually used only in short bursts to provide increased power upon demand after which the battery is recharged using the internal combustion engine or via feedback from a regenerative braking process.
There are further variations within these two broad categories. One variation is made between systems which are “charge depleting” in the one case and “charge sustaining” in another case. In the charge depleting system, the battery charge is gradually depleted during use of the system and the battery thus has to be recharged periodically from an external power source, such as by means of connection to public utility power. In the charge sustaining system, the battery is recharged during use in the vehicle, through regenerative braking and also by means of electric power supplied from the a generator powered by the internal combustion engine so that the charge of the battery is maintained during operation.
There are many different types of systems that fall within the categories of “charge depleting” and “charge sustaining” and there are thus a number of variations within the foregoing examples which have been simplified for purposes of a general explanation of the different types. However, it is to be noted in general that systems which are of the “charge depleting” type typically require a battery which has a higher charge capacity (and thus a higher specific energy) than those which are of the “charge sustaining” type if a commercially acceptable driving range (miles between recharge) is to be attained in operation.
A key enabling technology for HEVs is having an energy storage system having a high energy density while at the same time being capable of providing very high power. Such a system allows for recapture of energy from braking currents at very high efficiency while enabling the design of a smaller battery pack.
A typical auxiliary battery pack as used in HEV applications is a nickel metal hydride battery pack. In general, nickel-metal hydride (Ni—MH) cells utilize a negative electrode comprising a metal hydride active material that is capable of the reversible electrochemical storage of hydrogen. Examples of metal hydride materials are provided in U.S. Pat. Nos. 4,551,400, 4,728,586, and 5,536,591 the disclosures of which are incorporated by reference herein. The positive electrode of the nickel-metal hydride cell comprises a nickel hydroxide active material. The negative and positive electrodes are spaced apart in the alkaline electrolyte.
Upon application of an electrical current across a Ni—MH cell, the Ni—MH material of the negative electrode is charged by the absorption of hydrogen formed by electrochemical water discharge reaction and the electrochemical generation of hydroxyl ions:                     M        +                              H            2                    ⁢          O                +        e        -                                            ⟷              charge                        discharge                    ⁢          MH                +                  OH          -                                    (        1        )            The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron.
The charging process for a nickel hydroxide positive electrode in an alkaline electrochemical cell is governed by the following reaction:                                           Ni            ⁡                          (              OH              )                                2                +        OH        -                                            ⟷              charge                        discharge                    ⁢          NiOOH                +                              H            2                    ⁢          O                +        e        -                            (        2        )            
After the first charge of the electrochemical cell, the nickel hydroxide is oxidized to form nickel oxyhydroxide. During discharge of the electrochemical cell, the nickel oxyhydroxide is reduced to form beta nickel hydroxide as shown by the following reaction:                     NiOOH        +                              H            2                    ⁢          O                +                              e            -                    ⁢                                    ⟷              discharge                        charge                    ⁢                                    Ni              ⁡                              (                OH                )                                      2                          +                              OH            -                    .                                    (        3        )            
While the inclusion of an auxiliary battery pack working in conjunction with a fuel cell has many advantages for powering vehicles, such systems still have disadvantages upon implementation in a vehicle. The disadvantages of including a battery along with the fuel cell may include increased weight, space, terminals, inter cell connects, cost, maintenance, etc. Improvements in these areas will help fuel cells to become the standard source of power for vehicles and many other applications.